Switching arrangement in duplex loud speaking systems



Sept. 23, 1947. 0, TSCHUMI 2,427,983

SWITCHING ARRANGEMENT IN DUPLEX LOUD SPEAKING SYSTEMS Filed July 16, 1945 o 6] vezziz O, sclzuppzl Patented Sept. 23, 1947 SWITCHING ARRANGEMENT IN DUPLEX LOUD SPEAKING SYSTEMS Otto Tschumi, Soleure, Switzerland, assignor to Autophon Aktiengesellschaft, Soleure, Switzerland, a joint-stock company of Switzerland Application July 16, 1945, Serial No. 605,403 In Switzerland March 9, 1944 Section 1, Public Law 690,August 8, 1946. Patent expires March 9, 1964 Claims.

In an effort to simplify greatly the apparatus used at each station in duplex systems for loud trafilc and especially to make it independent of net work connections, it had been hitherto usual to feed the amplifying valves from a central supply source over two lines each containing twin cables. All projects exploited up to the present day have been constructed for the use of direct current supply. The solution however is inefiicient on account of the anode potential requirements of the amplifier being too high with respect to the standard tension authorized over long distant lines, on the one hand, and on account of the correct heating potential only being able to be obtained through resistances which obviously incur a potential drop, which in addition must be compensated for by the use of further resistances entailing still further working losses, on the other hand.

This disadvantage can be avoided to a great extent by the use of alternating current mains in place of direct current mains. In this way it is possible to furnish the sets at each station from mains at the required potential of the valves over a twin cable line at a tension quite independent of these requirements, the line tension not exceeding the voltage authorized for longdistant lines: this owing to the possibility of transforming the alternating current supply at the station mains to anode potential requirements.

This manner of feeding does away with all potential losses due to resistances in the anode and cathode circuits in the case of direct current supply. I

The switching arrangement according to the invention for duplex systems for loud traffic with twin cable lines from a central supply to amplifiers at each station, respectively, is characterized in that the supply current to the amplifiers over the twin cable connecting lines is alternating at a frequency differing sensibly from the voice-frequency.

In order to avoid any risk of induction in the amplifier circuit by the supply current or, vice versa, of induction in the supply current circuit by the amplifier current, the switching is proand supply circuits, respectively, arranged in such a manner as to let through voice-frequency currents in the amplifier circuit, whereas the supply filter is completely impervious to the same frequency range. Moreover, a still further arrangement could be devised: By combining the eifect of a transformer in one line and a filter in the other, i. e., by replacing one or the other of the transformers in the described transformer arrangement above by a frequency filter.

The drawing represents diagramatically, and

by way of example, a switching arrangement according to the invention. S denotes a centralielectric supply unit, TS a subscribers set. The second set TSI is not shown, being similar in construction to TS. TS and TSI are connected to a, b and al, bl, respectively. The supply unit consists of a transformer TR connected on the one side to an alternating current supply main which can be interrupted by a switch 1. The other side is connected to the outgoing supply linesat the subscribers sets. The transformer winding is tapped in the middle and connected to earth on the one side of the tapping according to alternatingcurrent requirements through, a condenserk, on the other side, through a time-delay relay in series with a source of direct current B. a. b denote the terminals on the subscribers set TS and on the supply unit s of the line, al, bl representing the same disposition with respect to the non-illustrated line and set TSl.

The subscribers station consists of a linetransformer LU, a supply current circuit-and amplifier circuit.

Both terminals a, b at the. station are connected to the oppositely wound primary coils of the line-transformer LU. The two secondary vided with independent line and supply transformers in such a manner that the effects of the supply current in the first, of the voice-current in the second are completely annulled Within the respective apparatus. The same end may be attained by providing filters in the amplifier coils of the line-transformer lead over the grid and anode transformer primary windings to the amplifying valve V, respectively. j

The supply current circuit is made up of a transformer ST, the primary winding of which is tapped in the middle, the tapping; being earthed on the one hand, through a relay W in series with a condenser 0 according to alternating current requirements, on the other hand, directly over a key T which is open whenin normal position. The secondary side of the supply current transformer ST is tapped in such a manner so as to permit the connection to conduct the current directly over rectifiers to the anode and cathode of the amplifying valve V.

The amplifier circuit consists of an amplifying valve V, a grid transformer EV, and an anode transformer AT which, dependent on the direction of transmission is connected over the two directional switches wt and 102 to the linetransformer LU or to the voice-current transformer LM.

The drawing illustrates the normal switchin position.

Both subscribers stations are switched according to the diagram for receiving (corresponding positions of the switches ml and 1.02) and the electro-acoustic transformer LM is in loud speaker working position, the contact r in the electric supply unit being open.

Let us say the subscriber at TS wants to call the station TSl; all he has to do is to press the key T and hold it down. As a result the circuit, composed of the two sides of the secondary coil of transformer TR, the two lines a, b, the linetransformer LU, both sides of the primary coil of the supply-transformer ST and the closed key T, is fed by a direct current source B. Relay R is energised and controls in its turn its contact T which closes the alternating current circuit formed by the supply-source so that alternating voltage is induced on the subscriber side of the supply transformer. The voltage is again transformed and converted at the subscribers stations to correspond to the required electric conditions of the anode and. cathode of the amplifying valves. In this way the key T sets up connections of the amplifying sets in both stations making them ready for operation.

The aforementioned alternating current has a being open, relay Wl which corresponds to W at TS, is energised, controlling the contacts wl andw2. TS! is ready for transmission.

The primary coils of the line-transformer LU being oppositely wound, the leads parallel, the induced effects neutralize one another and in this way the secondary coils are freed from supply interference. However, a frequency filter is provided at F for securitys sake, i. e., between the secondary side, of the said transformer and the grid-transformer Eu, thus impeding leakage of any currents in this circuit other than the voice-currents. In the same manner, the voicecurrent is freed from supply-current interference by arranging the split primary winding so that the current flowing through one part is opposite in direction to the current flowing through the other, 7

This special arrangement of the supply and voice-current transformers could be replaced by two filter arrangements, one of which gives free passage to voice-current-frequencies, whereas the other impedes them. Should the first of these filter arrangements be connected in the amplifier circuit, the other in the supply circuit, there could be no danger of mutual interference between the two circuits. In a similar way the same end may be attained by combining or the supply transformer arrangement described above with a filter in the line circuit or else the line transformer arrangement described above with a filter in the supply circuit. It is thus possible in different ways to separate completely the supply and line currents.

Should it be desired to change the direction of transmission, it is only necessary for the calling subscriber to let go the key T during a small interval of time. To this end the key is provided with a time regulating device. On interruption .4 the direct current circuit is opened thus deenergising relay R. which operates after a small lapse of time only. On the other hand relay W is no longer short-circuited, so that during the operating period of relay B, it controls the switches wl and 202. At conclusion of the time delay, the contact 1' opens and interrupts the alternating current supply and the set is dead. It is clear that on opening the key T the calling set TS has been switched over to the transmitting position and the supply current cut oif. At conclusion of the short interruption which should correspond to the lapse of time between the deenergisation of relay R. and its operation, should the key be now pressed down once more, then in addition to the closing of the supply source circuit, WI at TS! is energised controlling in its turn the changing over to receiving position of the switching apparatus. The direction of transmission has been changed: The operation can be indefinitely repeated at will.

At conclusion of the conversation the key T of calling set TS is let go and left open so that the supply current is again interrupted in the manner already described. Further the supply transformer is fitted up with a device so as to bring the switching apparatus ml and 102 at conclusion of the conversation back to its normal receiving position, in the case of it not being already so.

It is moreover possible to arrange the supply source in such a manner so as to make it possible for the change over operation to be done as before by means of the key T at the called set.

Should the supply source be used in connection with an automatic central exchange, controlled by selectors and relays, any number of subscribers can be looped in. In this case the key T is replaced by a dial switch.

What I claim is:

1. In an installation, an alternating voice current line, a duplex system comprising, a central supply unit for supplying alternating current with another frequency than the voice current in said line, a, subscribers station connected to said line, a two-wire connection between said central supply and said subscribers station, and a loudspeaker-amplifier arranged in said subscribers station to be fed through said two-wire connection from said central supply.

2. In an installation according to claim 1, a line transformer in connection with said central supply unit, and a supply transformer in connection with said voice current line, said transformers being arranged so that said line transformer is traversed by the supply current and said supply transformer by the voice current in such a way that the effects of these currents are annulled within said transformers.

3. In an installation according to claim 1, an amplifier circuit comprising said loudspeaker amplifier, a supply circuit comprising said central supply unit, first filters in said amplifier circuit with a permeability limited to the voice frequency range, and second filters in said supply circuit, impervious to the voice frequenc range.

4. In an installation according to claim 1, a line transformer in connection with said central supply unit in such a way that the eifects of the supply current within said line transformer are annulled, a supply circuit comprisin said central supply unit, and filters in said supply circuit, impervious to the voice frequency range.

5. In an installation according to claim 1, a supply transformer in such a connection with 5 6 said voice current line that the effects of the REFERENCES CITED voice Current trawl-sing Said supply transformer The following references are of record in the are annulled within the latter, an amplifier circuit comprising said loudspeaker-amplifier, 111- me of thls patent" ters in said amplifier circuit with a permeability 5 UNITED STATES PATENTS limited to the voice frequency range. Number Name t 1,987,320 Bellinger Jan. 8, 1935 OTTO TSCHUMI- 2,102,903 Leveque Dec. 21, 1937 

